A very interesting place to visit. It's understandable why this is an important place for the Norwegians.
Pros: impressive and gripping.
Cons: there's very little information except the little map you can buy in the cafe. One can book a guided tour, which we didn't, but I'm sure this would add even more to the experience.
But if you have...
I would have paid to enter this old fort / battle heritage site. While you have to understand Norwegian to read the history and posted signs, it is worth the drive and a great spot for families who like to explore.
My 6 year old wanted to explore every inch of the tunnel system, while my 3 year was slightly...